This invention relates to a bridge circuit serving as a part of a detector. More particularly, the invention relates to a bridge circuit with reduced drift due to variations in the environmental temperature and detectors of various kinds incorporating such a bridge circuit.
It has been known to make use of the changes in the resistance of a resistor according to variations in various measurable physical quantities such as pressure, temperature and degree of vacuum in forming a bridge circuit by using such a resistor, as well as to provide detectors of various kinds, such as pressure sensors and detectors of thermal conductivity for a gas chromatograph, a strain gauge, a vacuum gauge or a temperature detector, for detecting physical variables by using such a circuit.
FIG. 1 shows the structure of a pressure sensor of a gas chromatograph, as an example of detector using such a bridge circuit including a pressure-sensitive resistor 1 enclosed inside a sealed vessel into which a gas can be introduced to have its pressure measured. This pressure-sensitive resistor 1 is connected with three other resistors 2, 3 and 4 to together form a quadrangle. A constant current circuit 5 is connected to the junction between the resistors 1 and 4, an amplifier circuit 6 has one of its input terminals connected to the junction indicated by numeral 9 between the resistors 1 and 2 and the other of its input terminals connected to the junction indicated by numeral 8 between the resistors 3 and 4, and a voltage measuring circuit 7 is connected to the output terminal of the amplifier circuit 6. The constant current circuit 5 is for passing a constant current of several mA through the resistors 1, 2, 3 and 4 forming a Wheatstone bridge and the voltage difference between the two voltage measurement points 8 and 9 is amplified by the amplifier circuit 6 and detected by the voltage measuring circuit 7.
The pressure-sensitive resistor 1 and the other resistors 2, 3 and 4 are adjusted such that they all have the same resistance when the pressure-sensitive resistor 1 is under a standard condition not subjected to a pressure to be measured. In other words, the voltage difference between the two voltage measurement points 8 and 9 is nearly zero under such a normal condition and hence the voltage measured by the voltage measuring circuit 7 is also zero.
When the pressure-sensitive resistor 1 is subjected to a pressure, it undergoes a strain due to the applied pressure. The pressure-sensitive resistor 1 is made of a material with the property of changing its resistance when strained. As the resistance of the pressure-sensitive resistor 1 changes due to a change in pressure, a non-zero voltage difference appears between the measurement points 8 and 9 and this voltage difference is inputted through the amplifier circuit 6 to the measurement circuit 7 and is thereby measured.
Although a pressure-sensitive resistor is connected with three other resistors as described above to form a pressure sensor, the resistance of a resistor is affected not only by pressure but also by other physical variables such as temperature and tension. Thus, if a resistor is used such that such other physical variables can be measured, it is possible to form detectors of such other physical variables such as a temperature detector and a strain gauge. Japanese Patent Publication Tokkai 9-236592 disclosed a resistor placed inside a sealed container used as a detector of thermal conductivity for a gas chromatograph by heating it, introducing a sample-containing gas into the container and making use of the fact that the temperature of the resistor changes according to the thermal conductivity of the gas.
One of the problems of prior art pressure sensors of the type described above was that the measured pressure varied, depending on the environmental temperature. In the pressure sensor described above with reference to FIG. 1, for example, the pressure-sensitive resistor 1 was used for detecting changes in a target gas but if the temperature of the target gas changes as well as its pressure, the change in resistance due to the change in pressure and that due to the change in temperature are superposed and hence the change in pressure could not be measured accurately. Moreover, the resistance of the pressure-sensitive resistor changes due to changes in temperature even if the pressure remains constant, giving rise to the phenomenon of drift in the output.
It is therefore an object of this invention to provide an improved bridge circuit and a sensor using such an improved bridge circuit with which the effect of drift due to changes in temperature can be eliminated and the changes in the physical variable originally intended to be measured can be accurately measured.
A bridge circuit embodying this invention, with which the above and other objects can be accomplished, may be characterize not only as comprising four resistors forming a Wheatstone bridge, one of the resistors being set at a position for measuring a desired physical variable and the bridge being adapted to output a voltage imbalance due to a change in the physical variable, but also as being provided with voltage measuring means for measuring the voltage differences between the two pairs of mutually opposite junctions of the resistors forming the Wheatstone bridge and a means for calculating the effects of temperature from these measured voltage differences to make a correction.